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1
In a very large nonstick skillet, heat 2 tablespoons of the olive oil until shimmering.
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2
Add the potatoes in a slightly overlapping layer and cook over moderately high heat, stirring occasionally, until golden and nearly cooked through, about 8 minutes.
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3
Add the onion, season with salt and pepper and cook, stirring occasionally, until tender, about 5 minutes longer.
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4
Add 1/2 tablespoon of the vinegar and gently toss to combine.
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5
Cover loosely and keep warm.
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6
Meanwhile, in another large skillet, heat the remaining 1 tablespoon of olive oil until shimmering.
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7
Season the steaks with salt and pepper and cook them over moderately high heat, turning occasionally, until browned and crusty, 8 to 10 minutes for medium rare.
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8
Transfer the steaks to a serving platter and keep warm.
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9
Add 1 tablespoon of the butter and the shallots to the skillet and cook over moderate heat until softened, about 5 minutes.
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10
Add the remaining 1 1/2 tablespoons of vinegar and cook until evaporated, about 30 seconds.
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11
Add the wine and cook until reduced by two-thirds, about 2 minutes.
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12
Remove from the heat and swirl in the remaining 1 tablespoon of butter.
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13
Add the parsley and any accumulated meat juices, season the sauce with salt and pepper and spoon over the meat.
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14
Serve the steaks at once with the potatoes.
